In FIG. 14 a prior art MOSFET 100 is shown as known from U.S. Pat. No. 7,074,643 B2. The prior art MOSFET 100 is made of an n+ silicon carbide (SiC) substrate 80 and comprises between a first main side 20 and a second main side 22 an n− doped drift layer 2. On the first main side 20, two n++ doped source regions 3, 3′ are arranged, each which is separated from the drift layer 2 in lateral direction (i.e. in a direction parallel to the first main side 20) by a p doped channel layer 4, 4′ and on a side opposite to the first main side 20 by a p+ well layer 5, 5′, which is higher doped than the channel layer 4, 4′. In between such two source regions 3, 3′ surrounded by a channel layer 4, 4′ and a well layer 5, 5′, a p++ doped contact layer 65 is arranged, which laterally extends to the source regions. Due to its high doping concentration the p++doped contact layer provides a good ohmic contact to a first main electrode 9 (source electrode). The contact layer 65 is a shallow layer spatially (i.e. in depth direction, which direction is vertical to the first main side 20) extending to a contact layer depth 67, which is less deep than the well layers 5, 5′, but electrically and mechanically contacting the well layers 5, 5′ in order to connect the well layers 5, 5′ to the source electrode 9. The contact layer 65 overlaps with the source regions 3, 3′ the channel layers 4, 4′, so that the contact layer 65 is the only p doped layer in contact with the first main electrode 9.
JP 2010 267762 A describes a MOSFET which comprises on the first main side a highly p doped plug and a lower doped well layer below an n source layer and a channel layer, which surround the source regions. First the well layer is formed, followed by the channel layer with a new, wide mask. Afterwards the p plug is formed with another mask and then the n source layers are formed with a fourth mask. As four different masks are used, this method is susceptible to mask misalignments. Furthermore, angled implantation is used for forming the layers, which makes the implantation more difficult. As the n source layers and the p plug have a comparable doping concentration, there is a danger in the transition area between the two layers of compensated charges, i.e. there may be no dominating charge so that there is an undesired neutralized zone.